In a typical iron production process, a blast furnace is used to reduce iron ore to liquid iron for subsequent processing. A typical blast furnace 10, shown in FIG. 1, can be as high as 100 feet and have a diameter of 50 feet. Inside a steel shell 11 of the furnace 10, three-foot-thick refractory carbon blocks 14 form a hearth-wall liner providing thermal insulation between the molten iron (not shown) and the shell 11.
When the furnace 10 is operating, a burden is fed into the top of the furnace 10. The burden typically includes iron ore, coke and limestone. The iron ore provides iron which serves as the predominant component of steel. The coke combusts to provide the heat required for smelting. Moreover, the coke also supplies needed carbon and carbon dioxide. The limestone serves as a flux to form a fluid slag that can be readily drained from the hearth 12.
As the burden is fed to the furnace 10, it fills the stack 16 as a solid aggregate. The burden is then forced through the stack 16 down into an inverted conical section, known as the bosh 18, where melting starts. A blast of heated air and fuel are introduced through openings 20 at the bottom of the bosh 18, just above the hearth 12 to melt the burden. The resulting iron melt and slag accumulate in the hearth 12 to form a molten bath until drained through a tapping hole 22.
As iron is processed in the furnace 10, the carbon blocks 14 are gradually worn away or weakened due to erosion caused by the mechanical motion of molten iron and also due to infiltration of the molten iron into cracks which develop in the blocks 14. Infiltration and thinning of the carbon blocks 14 has conventionally required that the entire furnace 10 be shut down and relined approximately every six years. For a large furnace, relining requires three to four months downtime of the furnace and consequent loss of production. The cost of relining and shutdown can be $120 million or more.
If relining is delayed, the mechanical integrity of the blocks 14 can fail catastrophically allowing the molten iron to escape through the hearth-wall liner. Furnace failure can easily cost $5 million to $50 million, depending on the extent of damage. In recognition of this danger, blast furnace operators typically err on the conservative side and often replace the hearth-wall lining prematurely. In an effort to better assess the appropriate time to replace the hearth-wall liner, a variety of techniques have been used to evaluate its condition. A common technique uses thermocouples embedded into the hearth-wall liner at various locations. To augment the reliability of these measurements, the use of thermocouples is usually supplemented by periodically drilling through the hearth wall. Although the drilling damages the hearth, it nevertheless enhances the determination of hearth-wall liner thickness. Additionally, invasive optical methods have been developed to evaluate the inner surface of the hearth for wear from inside the furnace when the furnace is shut down and emptied.